In chromatography and other applications, a precisely-controlled continuous flow of liquid is often desirable. Positive displacement pumps offer the desired precision, but a single piston or other displacement element can only deliver liquid during a portion of its operating cycle and must fill with incoming fluid during the remainder of the cycle. To provide a continuous output, it is conventional to use two pistons and cylinders connected in parallel, with each one delivering liquid during half of the cycle. E.g., Abrahams et al. U.S. Pat. No. 3,855,129 and Swarthout U.S. Pat. No. 3,025,803. These pumps are relatively complicated. Four check valves are generally required: one at the inlet to each cylinder and one at each outlet. And because flow through the pump is necessarily divided upstream of and then joined downstream of the two cylinders, flow mixing can result between serial flow segments entering the pump. Further, in high pressure pumps employing two such cylinders, an air bubble entering one cylinder can remain trapped therein, effectively shutting down that piston and cylinder. This occurs because the other piston maintains pressure in the outlet line and thereby keeps the outlet check valve on the first cylinder closed. The first cylinder cannot generate enough pressure, because of the bubble, to open the check valve. For this reason, such pumps require careful initial bleeding and continuous monitoring thereafter. Another pump that delivers a relatively continuous flow is the very old and well known design for a hand water pump, in which water is delivered on both up and down strokes of the pump handle. There, a single piston with a step down in cross section moves through two adjacent volumes of water. Water stored in the upper volume during the down stroke is expelled on the up stroke.
As a further step towards providing continuous, smooth flow delivery in a two-cylinder pump, it is known to provide a pulse dampener downstream of or between the cylinders. Liquid stored in the dampener during high flow (and high pressure) periods of a cycle is released during transitions between cylinders when flow displaced from the cylinders is momentarily low. A length of flattened tubing that expands in cross section at high pressures is typically used for liquid storage to reduce mixing between serial flow segments. To store the flattened tubing compactly, it is conventional to bend it into many convolutions and store it inside a can filled with potting material (e.g., Waters Associates, Inc. Model 6000A pump). The tubing has also been bent into a helix and stored inside a cylindrical can.